In a time-of-flight mass spectrometer (TOFMS), various ions derived from a sample are ejected from an ion ejector, and the time of flight required for each ion to fly a certain flight distance is measured. Each ion flies at a speed according to its mass-to-charge ratio m/z. Accordingly, the above-mentioned time of flight corresponds to the mass-to-charge ratio of the ion, and the mass-to-charge ratio of the ion can be obtained based on its time of flight.
FIG. 14 is a schematic configuration diagram of a typical orthogonal acceleration TOFMS (hereinafter, it may be referred to as “OA-TOFMS”).
In FIG. 14, ions generated from a sample in an ion source (not shown) are introduced into an ion ejector 1 in the Z-axis direction, as shown by an arrow in FIG. 14. The ion ejector 1 includes a plate-shaped push-out electrode 11 and a grid-shaped extraction electrode 12, which are arranged to face each other. Based on control signals from a controller 6, an acceleration voltage generator 7 applies a predetermined level of high-voltage pulse to either the push-out electrode 11 or the extraction electrode 12, or between them, at a predetermined timing. By this operation, ions passing through the space between the push-out electrode 11 and the extraction electrode 12 are given acceleration energy in the X-axis direction and ejected from the ion ejector 1 into a flight space 2. The ions fly through the flight space 2 which has no electric field, and then enter a reflector 3.
The reflector 3 includes a plurality of annular reflection electrodes 31 and a back plate 32. A predetermined direct voltage is applied to each of the reflection electrodes 31 and the back plate 32 from a reflection voltage generator 8. A reflective electric field is thereby formed within the space surrounded by the reflection electrodes 31. The ions are reflected by this electric field, and once more fly through the flight space 2, to eventually reach a detector 4. The detector 4 generates ion-intensity signals according to the amount of ions that have reached the detector 4, and sends those signals to a data processor 5. The data processor 5 prepares a time-of-flight spectrum that shows the relationship between the time of flight and the ion-intensity signal, with the point in time of the ejection of the ions from the ion ejector 1 defined as the time-of-flight value of zero, and converts the time of flight to mass-to-charge ratio based on prepared mass calibration information, so as to calculate a mass spectrum.
When ions are to be ejected from the ion ejector 1 of the above-mentioned OA-TOFMS, a high-voltage pulse on the order of kV with a short duration needs to be applied to the push-out electrode 11 and the extraction electrode 12. For generating such a high-voltage pulse, a power supply device as disclosed in Patent Literature 1 (it is referred to as a “pulsar power source” in this document) has been conventionally used.
The power supply device includes: a pulse generator for generating a pulse signal for controlling the timing of the generation of the high-voltage pulse; a pulse transformer for transmitting the pulse signal from a control-system circuit to a power-system circuit while electrically insulating the control circuit that operates with a low voltage from the power circuit that operates with a high voltage; a driving circuit connected to the secondary winding of the transformer; a high-voltage circuit for generating a high direct-current voltage, and a switching element employing metal-oxide-semiconductor field-effect transistors (MOSFET) to generate a voltage pulse by turning on and off the direct-current voltage generated by the high-voltage circuit according to a control voltage provided through the driving circuit. Such circuits are not limited to TOFMSs; they are commonly used for generating high-voltage pulses (see Patent Literatures 2, 3, and others).
In an LC-TOFMS in which a liquid chromatograph (LC) is provided in the previous stage of the OA-TOFMS that includes an atmospheric pressure ion source, such as an electrospray ion source, it is necessary to detect, without omission, various substances contained in a sample liquid continuously introduced into the atmospheric pressure ion source of the TOFMS from the exit port of the column in the LC. To this end, a measurement operation that covers a predetermined length of time is repeatedly performed with a predetermined period in the TOFMS. The longer the repetition period of the measurement is, the wider the time interval becomes between the measurement points on a chromatogram to be created. This lowers the accuracy of the shape of a peak waveform of a target substance and deteriorates the performance of the quantitative measurement. For minimizing the time interval between the measurement points on the chromatogram, it has been common to control the device so that a relatively short measurement period is set in a measurement of ions that have low mass-to-charge ratios and short times of flight, while a relatively long measurement period is set in a measurement of ions that have high mass-to-charge ratios and long times of flight.
For example, the control is performed in such a manner that the measurement period is set to 125 [μs] for ions with low mass-to-charge ratios within a range of m/z 2000 or less, to 250 [μs] for ions with medium mass-to-charge ratios within a range of m/z 2000 to 10000, and to 500 [μs] for ions with high mass-to-charge ratios within a range of m/z 10000 to 40000.
Such a change in the measurement period can be achieved by changing the time interval of the generation of the high-voltage pulse to be applied to the push-out electrode 11 and the extraction electrode 12 of the ion ejector 1. In other words, even when the measurement period is changed, parameters other than the time interval of the generation of the high-voltage pulse, such as a pulse width (pulse application period), are unchanged irrespective of the measurement period.
In a power supply device for generating a high-voltage pulse as mentioned above, a slight delay in time inevitably occurs between the point in time of the rising of the pulse signal fed to the pulse transformer and the point in time of the rising of the high-voltage pulse outputted from the power supply device. In principle, the delay in time should be constant and unaffected by the measurement period as long as the voltage value (pulse height) of the high-voltage pulse is the same. However, the present inventor has found that a temporal fluctuation occurs in the rising of the high-voltage pulse generated by the power supply device in a conventional OA-TOFMS when the measurement period is changed.
In TOFMS, the time of flight of each ion is measured from the point in time where the ion is ejected or accelerated. Accordingly, in order to enhance the accuracy in the measurement of the mass-to-charge ratio, the point in time of the initiation of the time-of-flight measurement needs to coincide with the timing of the actual application of the high-voltage pulse to the push-out electrode or the like as much as possible. If the aforementioned temporal fluctuation occurs in the rising of the high-voltage pulse due to the change in the measurement period, the temporal fluctuation causes a time discrepancy between the point in time of the initiation of the measurement and that of the ejection of the ion. This discrepancy causes a corresponding time-of-flight difference among ions having the same mass-to-charge ratio, and a mass discrepancy occurs. Accordingly, changing the measurement period deteriorates mass accuracy. To avoid this deterioration, mass calibration information that shows a correspondence relationship between the time of flight and the accurate mass-to-charge ratio may be used for each of the different measurement periods for the conversion of the time of flight into mass-to-charge ratio. Preparation of the mass compensation information requires an actual measurement of a standard sample containing a substance having an accurately known mass-to-charge ratio. Therefore, preparing mass compensation information for every measurement period is an extremely troublesome and time-consuming job.